Conventional paper for electrophotographic high-speed machines is comprised of accordion-fold continuous form paper, and continuous form paper which is fed into the printer via rollers. In recent years, however, electrophotographic colour copy machine and colour printers which use cut-sheet paper are now required to have a throughput speed of 60 to 100 pages per minute.
The main advantages to using cut-sheet paper (hereafter “sheet-fed paper”) are that it is easy to change the paper type and size, and that there are no wasted offcuts when bookbinding since there is no need to cut the paper. Sheet-fed paper is suitable for “on-demand” small lot printing—which handles manuals, self-published and other materials in the order of a few thousand units or less—where it can comprehensively address customers' requests.
However, because of the disadvantages of copy machines and printers using sheet-fed paper, it is difficult to increase the printing speed in comparison to continuous form paper. In sheet-fed copy machines and printers, paper is passed along by running it between rollers or belts rather than by pinching the paper with claws as sheet-fed offset printers do. Because of this, paper with low stiffness has poor throughput performance. The paper is not easily transferred between rollers and jams easily, making continuous printing difficult.
Conventionally, woodfree paper has been used with high-speed, sheet-fed electrophotographic copy machines and printers as an adequate paper. However, as described previously, in order to meet wide and varied customer's demands arising from the spread of self-publishing and the like, there is a need to develop a coated paper with superior printability which will enable continuous printing on high-speed colour copy machines and colour printers. Further, when using the paper for book leaves, weight (basis weight) and thickness are important. Paper can reach a considerable weight if aggregated many times over. In particular, coated paper has a high basis weight. Since books with coated paper leaves become extremely heavy, there is a need for a light and thin coated paper. Generally, however, reducing the weight of the paper results in decrease of the paper thickness and reduces the stiffness, giving rise to a tendency for running performance failures such as a poor paper feed or jams. This is particularly the case for electrophotographic copy machines and printers.
In terms of pigment-coated electrophotographic paper, electrophotographic transfer papers which specify the center line average roughness, the surface electrical resistance or the static friction coefficient between paper sheets have already been disclosed (Patent Documents 1-3). This is after the base paper has been given a coating layer that includes a pigment with a specific particle size and a specific adhesive agent, and then has been subjected to smoothing. However, the image quality and throughput performance of this transfer paper has been far from adequate. Further, a method has been disclosed that enhances the printed image and throughput performance by specifying the basis weight, the surface roughness of the coating layer and the stiffness (Patent Document 4). However, not only an image quality but also high-speed running performance has been insufficient.
PATENT DOCUMENT 1: Unexamined Patent Application Publication
TokkaiSho62-198875
PATENT DOCUMENT 2: Unexamined Patent Application Publication
TokkaiSho62-198876
PATENT DOCUMENT 3: Unexamined Patent Application Publication
TokkaiSho62-198877
PATENT DOCUMENT 4: Unexamined Patent Application Publication
Tokkai2000-172001
Generally, it is often the case that a coating layer of 5.0 g/m2 or more is created on one side of the base paper, after which the coated paper is manufactured by smoothing the surface via calendaring. The paper's stiffness is reduced as a result of being put under pressure during the calendaring process. Given this, one way of improving coated paper stiffness has been to improve the base paper.
For instance, the inclusion of paper strength additives such as polyacrylamide or starch improves paper stiffness. However, increasing the proportion of the additives than usual, the formation of the paper apt to deteriorate and, rather tends to reduce stiffness. Further, not only does the addition of large quantities of these cohesive and adhesive chemicals during the paper making process have tendency of the process defects but also increase of costs.
On the other hand, branched polyacrylamide having a relatively high molecular weight (approximately 2,500,000 to 4,000,000) is used as a paper strength improver additive. By using it as a surface coating agent, the stiffness is remarkably improved, since the large effect of binding pulp fibers existing in a paper surface together results in a suppression of its penetration into the paper. The branched polyacrylamide as an additive is manufactured from an acrylamide monomer, a cationic monomer such as quaternary and/or tertiary ammonium, and/or an anionic monomer containing a carboxyl group, and a cross-linking agent. However, it has high viscosity and shows a high spinnability. This leads to process and quality the problems including boiling during the coating process and the occurrence of a mist between the coating rollers which give rise to non-uniform coating.
Further, a method has been disclosed (Patent Document 5) for increasing paper stiffness by first coating the base paper with an acrylic resin that contains polyacrylamide using a size press and then preparing a coating layer that includes a pigment and an adhesive agent. However, the acrylic resin that is generally used as a paper surface coating agent has been unable to improve the stiffness sufficiently since it is a polymer with a relatively low molecular weight (approximately a several 100,000). When the molecular weight is increased the stiffness is improved, however, the coating to the base paper is hardly carried out since the viscosity of the coating fluid becomes extremely high.
As described above, obtaining a coated electrophotographic transfer paper having a coating layer containing a pigment and an adhesive agent, which has superior running performance, was difficult with conventional technology.
PATENT DOCUMENT 5: Unexamined Patent Application Publication
Tokkai2004-068242
Further, high-speed sheet-fed electrophotographic copy machines and printers are used in the printing of small lots less than several thousand units. Therefore, demand for electrophotographic transfer paper has increased, however, it still only occupies a very small amount of the paper stock used in printing.
In addition, on-demand printing does not always use high-speed, sheet-fed electrophotographic copy machines and printers. As with invoices, receipts and similar documents, sometimes the predetermined parts are printed in advance via offset printing while just the section with variable information is later added via on-demand printing. Further, the majority of printing houses that carry out on-demand printing also perform offset printing. Instead of paper specialized to on-demand printing, if they had multipurpose paper which could also be used for offset printing, then they could reduce the burden associated with managing paper stock and could reduce the amount of stock held. From this viewpoint, there is a need for electrophotographic transfer paper which also has superior offset printability.
However, the properties of electrophotographic transfer paper are inferior to ink adhesion property and surface strength which are required for offset printing paper. It has been difficult, therefore, to develop a coated paper which is: superior in terms of high-speed, sheet-fed electrophotographic high-speed throughput performance and printing property; has good printability in terms of ink adhesion property and surface strength which are necessary for offset printing; and which is superior in terms of both electrophotographic and offset printability, having satisfactory coating performance and productivity.